Waters proposed an identity-based encryption and signature scheme in standard model.Then Paterson and Schuldt proposed an identity-based signature scheme based on Waters' IBE.Zhang and Xu proposed an identity-based multi-signcryption scheme that using the above schemes.However

we show that Zhang-Xu's scheme is insecure under randomizing private key attack.Furthermore

we propose an improved identity-based multi-signcryption scheme in standard model.In our scheme

we divide the private keys into two parts

one is for decryption

the other is for signature

and the private key used for signature cannot be randomized.Moreover

our scheme can resist insider leak attack.